I haven't seen any comparisons between the FAudio Spring vs the FAudio Major, so I thought I would share my opinions given I have owned both for a while. I think it's important given the Spring uses a second-gen driver based on the Major's driver.
Both IEMs use single DDs with a double diaphragm design - one being made out of titanium, and the other one out of fiber. The main differences are that the Major's driver is a bit larger (10.5mm vs 10.2mm) and the Major's sound tube is made out of pure copper, while the Spring's sound tube is made out of stainless steel.
Sorry for the ugly photos
Despite using similar drivers, their sound is completely different. The Major is V-shaped with a slow-ish bass decay, rumbly subbass and an aggressive treble, while the Spring is mostly neutral with slightly elevated midbass and airy treble.
I struggled for a long time to find tips that I liked with the Major, and I almost gave up on it (I actually listed it for sale). However, I could always tell the mids on the Major were special. No matter what tips you use, you can just hear so much information and micro details in the mids with it. It's so detailed and technical, you could swear you're listening to a hybrid at times. I finally settled with the Sony EP-EX11 eartips (imagine a Final Type E but with similar feel to an Eletech Baroque) to tame the treble, and I can see why people loved it when it originally got released. It's almost sorcery how a single DD that's bassy can give you that much information. Also worth noting that the soundstage is mostly tall with decent width.
If I were to describe the Spring with a few words, I would say it sounds sweet, beautiful, and delicate. If I had to stay with only one IEM out of my collection, I would keep only the Spring. It's just so versatile, and it will play everything you throw at it well. The bass is not its main point, but it's fast and hits with authority. The mids don't throw as many details at you as the Major, but the beautifully tuned treble makes up for it. I also find it works with so many more eartips. Personally, I like it the most with the Tangzu Sancai Balanced, the Eletech Baroque, and the Spinfit CP100.
I feel like they don't really compete with each other, as they're different enough that you can keep both in your collection. I pick up the Major when I want a 'Hi-Fi' listening session with a highly dynamic sound that also shows me all of the hidden background voices, claps, whispers, etc. I pick up the Spring if I just want to relax and listen to some beautiful music.
BONUS: A photo of my FAudio Sample with a CanJam case

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